The invention relates generally to ground vibration testing of large flexible structures, such as aircraft. More specifically, the invention relates to a structural test soft support system that allows resulting rigid body test mode data to be separated, or decoupled, from first flexible mode test data.
The purpose of a ground vibration test is to verify complex vibration analyses, or alternately, to determine important vibration characteristics of a structure. Prior testing has shown that it is very likely that a ground vibration test will produce results which will be contaminated by a series of unknown non-linearities within the structure and, often more significantly, by the effects of the test support system non-linearities, stiffness and mass.
A successful ground vibration test of a flexible structure requires an analytically determinant definition of the test structure supports referred to as test boundary conditions. This is an absolute requirement because the effects of the test support system must be removed in order to allow an understanding of the test structure alone.
For aircraft ground vibration tests, an ideal xe2x80x9csoft support systemxe2x80x9d would position the aircraft with a boundary condition that is consistent with the simulation of an aircraft in flight. Additionally, the ideal support system would support the aircraft so that the test rigid body mode frequencies, defined as the interaction frequencies of the aircraft structural weight and inertia with the soft support system stiffness, are well below the first flexible modes of the structure. The most important flexible structural modes that contribute to potential critical aeroelastic phenomena have frequencies which occur just above the rigid body mode frequencies. The engineering objective is to tune the highest occurring rigid body mode, using frequency as a guide, to be separated from the lowest flexible structural mode frequency.
At least one known soft support system for aircraft ground vibration tests has included various spring and spring supported platforms that have been placed below each landing gear after jacking up the aircraft to provide clearance below the landing gear. The platforms are interfaced with the landing gear and then released from the supporting or grounding devices. These soft support systems have been fairly successful, but have been limited by three factors. One limiting factor is the excessive mass of the soft support system due to the geometry and strength design requirements. Known soft support systems have platforms that are placed under each landing gear tire assembly. The platforms are quite large and typically weigh several thousand pounds. As the mass of the soft support system approaches a noticeable percentage of the aircraft weight, the tested aircraft dynamic behavior is altered and contamination occurs in the data. This makes analysis model verification using the test results more time consuming to correlate, and more ambiguous, and often not possible to interpret correctly.
A second limiting factor of known soft support systems is the difficulties incurred due to tire stiffness nonlinearities. With the tires under load and in series with the soft support system during the test, an amplitude varying nonlinear stiffness is introduced into the test results. Nonlinear effects are very difficult to model using known analysis techniques. It is very important to make the test configuration and supports as linear as possible so that analysis model verification can be performed.
A third limiting factor of known soft support systems is an adverse effect on test flow time due to the significant time required for soft support system setup. In known soft support systems a significant amount of time consumption occurs during set-up of the soft support system. When the entire aircraft is jacked up sufficiently to place the platforms under the tires, time consuming diligent care must be taken so that the platforms are placed properly. It is not uncommon to have to re-jack the aircraft to improve the location of the platforms during a test. Therefore, data acquisition time is often compromised by time lost during soft support system placement.
The standard approach is to allow enough time for the setup of the system prior to dedicated test time, and to allow additional time during the test to analyze the data to ensure there is no contamination in the data due to soft support related difficulties. Thus, typical known soft support system approaches are conservative and time consuming. Time is very expensive and the importance of reducing the risk of acquiring contaminated or incorrect data during a certification related test can not be overemphasized.
Other known soft support system approaches have utilized methods that reduce the stiffness of the boundary condition by running a single tire from each landing gear up on a ramp. The tire pressure is then reduced to get as soft a system as possible while on tires. The overloaded tire is significantly non-linear and is problematic to define, thereby making the results from this type of test difficult to interpret. Often an engineer will spend thousands of hours hoping to verify the model. Due to the nonlinearities the results are often ambiguous and leave the certification analysis validation of the aircraft in question.
Therefore, it would be desirable to provide a soft support system that eliminates the risks to test data associated with known ground vibration test support systems. Specifically, it would be desirable to provide a structural test soft support system that supports a flexible structure, i.e. an aircraft, such that testing produces data wherein the undesirable test rigid body mode data caused by the test support structure is well below the desirable first flexible mode data of the structure under test, thereby allowing the rigid body mode data to be decoupled from the first flexible mode data.
In one preferred embodiment, the present invention provides a support system is provided for ground vibration testing of a large flexible structure. The system includes a plurality of light weight lifting mechanisms that impart an upward force sufficient to lift the flexible structure off the ground. The system also includes a plurality of lift beams coupled to the lifting mechanisms in a pendulous manner. Each lift beam attaches to the flexible structure at one of a plurality of designated jack points, and lifts the flexible structure off the ground when the upward force is imparted by the lifting mechanisms. Thus, the flexible structure is thereby pendulously suspended above the ground such that rigid body mode test data is decoupled from first flexible mode test data.
In another preferred embodiment, a method is provided for structurally testing a flexible structure using a soft support system. The system includes a plurality of light weight lifting mechanisms and a plurality of lift beams pendulously coupled to the lifting mechanisms. The method includes attaching the lift beams to the flexible structure at designated jack points, and pendulously suspending the flexible structure above the ground utilizing the lift beams such that rigid body mode test data is separated from first flexible mode test data.
In yet another preferred embodiment, a support system is provided for ground vibration testing of an aircraft having a plurality of land gears comprising a landing gear strut and a plurality of tires. The system includes a plurality of fluid pressure canisters capable of imparting an upward force sufficient to lift the aircraft at each landing gear strut. Additionally, the system includes a plurality of hanger beams having a first end coupled to one of the fluid pressure canisters and an opposing second end coupled to another of the fluid pressure canisters. Furthermore, the system includes a plurality of hanger rods coupled to the hanger beams such that a proximal end of each hanger rod is pendulously coupled to a respective hanger. Still further, the system includes a plurality of lift beams connected to the hanger rods such that one lift beam is connected to the distal ends of two hanger rods. Each lift beam attaches to one of the landing gears between the tires such that the aircraft is pendulously suspended above the ground when the fluid pressure canisters impart the upward force, thereby allowing test data to be produced such that rigid body mode test data can be separated, or decoupled, from first flexible mode test data.